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<title>Debug Information (GNU Compiler Collection (GCC) Internals)</title>

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<h3 class="section" id="Variable-Location-Debug-Information-in-RTL"><span>14.18 Variable Location Debug Information in RTL<a class="copiable-link" href="#Variable-Location-Debug-Information-in-RTL"> &para;</a></span></h3>
<a class="index-entry-id" id="index-Variable-Location-Debug-Information-in-RTL"></a>

<p>Variable tracking relies on <code class="code">MEM_EXPR</code> and <code class="code">REG_EXPR</code>
annotations to determine what user variables memory and register
references refer to.
</p>
<p>Variable tracking at assignments uses these notes only when they refer
to variables that live at fixed locations (e.g., addressable
variables, global non-automatic variables).  For variables whose
location may vary, it relies on the following types of notes.
</p>
<dl class="table">
<dt><a id="index-var_005flocation"></a><span><code class="code">(var_location:<var class="var">mode</var> <var class="var">var</var> <var class="var">exp</var> <var class="var">stat</var>)</code><a class="copiable-link" href="#index-var_005flocation"> &para;</a></span></dt>
<dd><p>Binds variable <code class="code">var</code>, a tree, to value <var class="var">exp</var>, an RTL
expression.  It appears only in <code class="code">NOTE_INSN_VAR_LOCATION</code> and
<code class="code">DEBUG_INSN</code>s, with slightly different meanings.  <var class="var">mode</var>, if
present, represents the mode of <var class="var">exp</var>, which is useful if it is a
modeless expression.  <var class="var">stat</var> is only meaningful in notes,
indicating whether the variable is known to be initialized or
uninitialized.
</p>
</dd>
<dt><a id="index-debug_005fexpr"></a><span><code class="code">(debug_expr:<var class="var">mode</var> <var class="var">decl</var>)</code><a class="copiable-link" href="#index-debug_005fexpr"> &para;</a></span></dt>
<dd><p>Stands for the value bound to the <code class="code">DEBUG_EXPR_DECL</code> <var class="var">decl</var>,
that points back to it, within value expressions in
<code class="code">VAR_LOCATION</code> nodes.
</p>
</dd>
<dt><a id="index-debug_005fimplicit_005fptr"></a><span><code class="code">(debug_implicit_ptr:<var class="var">mode</var> <var class="var">decl</var>)</code><a class="copiable-link" href="#index-debug_005fimplicit_005fptr"> &para;</a></span></dt>
<dd><p>Stands for the location of a <var class="var">decl</var> that is no longer addressable.
</p>
</dd>
<dt><a id="index-entry_005fvalue"></a><span><code class="code">(entry_value:<var class="var">mode</var> <var class="var">decl</var>)</code><a class="copiable-link" href="#index-entry_005fvalue"> &para;</a></span></dt>
<dd><p>Stands for the value a <var class="var">decl</var> had at the entry point of the
containing function.
</p>
</dd>
<dt><a id="index-debug_005fparameter_005fref"></a><span><code class="code">(debug_parameter_ref:<var class="var">mode</var> <var class="var">decl</var>)</code><a class="copiable-link" href="#index-debug_005fparameter_005fref"> &para;</a></span></dt>
<dd><p>Refers to a parameter that was completely optimized out.
</p>
</dd>
<dt><a id="index-debug_005fmarker"></a><span><code class="code">(debug_marker:<var class="var">mode</var>)</code><a class="copiable-link" href="#index-debug_005fmarker"> &para;</a></span></dt>
<dd><p>Marks a program location.  With <code class="code">VOIDmode</code>, it stands for the
beginning of a statement, a recommended inspection point logically after
all prior side effects, and before any subsequent side effects.  With
<code class="code">BLKmode</code>, it indicates an inline entry point: the lexical block
encoded in the <code class="code">INSN_LOCATION</code> is the enclosing block that encloses
the inlined function.
</p>
</dd>
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